This invention concerns improvements in or relating to cam mechanisms and relates more particularly to cam mechanisms which produce controlled linear motion from rotary motion.
Such mechanisms are used in lenses, and especially zoom lenses, where a lens element or group of lens elements is required to move axially responsively to angular movement of a rotatable member such as a focussing ring. In a zoom lens of the mechanically compensated type at least two lens elements or groups are required to move relatively to each other and relatively to fixed elements of the lens system in a particular manner. Such movements can be effected and controlled by use of suitable cams and cam followers. The cam is frequently provided by a ring with a groove or slot of basically helical (sometimes called spiral) form but with its curve adapted to provide the required movement, and is known as a scroll cam. The cam follower runs in the groove or slot so that relative rotation of the ring produces the required axial movement of the follower and hence of the lens element or group connected to the follower. Commonly the cam follower is a pin or the like which slides along the groove or slot but this can give rise to undesirably high friction. If low friction is required then precision bearings such as rollers can be used for the cam followers. However, this can give rise to problems of looseness and can make the system susceptible to backlash because the cam groove or slot must be wider than the diameter of the bearing or the follower will be unable to roll. As a practical matter, therefore, very tight tolerances have to be imposed, particularly on the width of the cam groove or slot, but even then some backlash can still occur.
Some prior proposals of cam mechanisms for zoom lenses are described in British Patent No. 1,066,502 which discloses a roller in a cam groove or slot, U.S. Pat. No. 4,386,829 which discloses a cam protuberance arrangement, and U.S. Pat. No. 4,465,344 which discloses a sliding cam follower.